The present invention relates to an ion generation method and an ion irradiation method, and more particularly, to an ion generation method and an ion irradiation method that are effective for formation of a shallow diffusion layer. The present invention also relates to a filament suitable for an ion generation apparatus.
The ion implantation method (ion irradiation method) is widely employed as a method of forming pn junction by introducing with an impurity such as boron (B), phosphorous (P), arsenic (As), or the like into a semiconductor substrate. In the ion implantation method, an impurity can be introduced into a desired portion by accurately controlling the concentration and the depth of the impurity.
As high integration of the ULSI is promoted and the element size is reduced, the importance in the formation of a shallow pn junction is increased. The above-mentioned ion implantation method is one of the doping techniques employed widely in the semiconductor device manufacturing process, and is employed for the formation of the pn junction by combination with a heat process (annealing) that is conventionally executed after the ion implantation.
However, formation of a shallow pn junction using B as the p-type dopant includes many difficult points as explained below. First, B, which is a light element, brings about the remarkable channeling tale at the time of ion implantation. For this reason, lowering the energy of the accelerated voltage for shallow introduction of B causes lowering of the effective dose due to the influence from reflection, spattering and the like. Otherwise, ions cannot be extracted when the voltage is too low in accordance with the apparatus performance. Further, B has a large diffusivity in silicon, and therefore, it brings about, for example, the short channel effect of a pMOS transistor.
Thus, a process using a heavy element such as gallium (Ga) and indium (In), which is also a p-type dopant similarly to B, is noticed. In an ion irradiation apparatus (an ion generation apparatus) executing the above-mentioned ion implantation method (ion irradiation method), generally, gas is introduced into an arc chamber, or either a solid or a liquid is sublimated and its vapor is introduced into an arc chamber, to execute the ionization of In.
In the case of the In ion implantation, a chloride (InCl3) is known as a solid source. The present inventor found the following problem in the p-type dopant ion implantation methods using the solid of this kind. That is, in the case of a chloride, chlorine corrodes metal members of the apparatus.
Particularly, the etching reaction is strong in the arc chamber and the ion source chamber, and therefore, a filament for emitting thermoelectrons is corroded. For this reason, the ionization of In cannot be stably executed and a long-time work is extremely difficult. Specifically, the work can be executed in an only short time ranging one to four hours and the use of the chloride is not practical.
On the other hand, an organic gas source such as trimethyl indium (TMI), triethyl indium (TEI) is known as a gas source. TMI, which has a vapor pressure at a normal temperature, is ionized in a support gas such as Ar, for example, and an In ion beam is extracted therefrom.
However, the present inventor also found the following problem in the p-type dopant ion implantation method using an organic gas source of this kind. An organic gas drastically reacts with oxygen or water and is therefore very dangerous. Even in a vacuum apparatus the organic gas source is too dangerous as an ion source of the ion irradiation apparatus, when the suction of atmosphere caused by vacuum leak and the like, filling of TMI/TEI, the ion source maintenance after use of TMI/TEI, and the like are considered.
Incidentally, the ion source chamber serving as a heart of the ion implantation apparatus (ion irradiation apparatus) is largely classified into the Freeman type using a hot electrode, the Bernas type and the microwave type using magnetron.
Next, a method of extracting ions by taking advantage of the hot electrode with this apparatus will be explained simply. Ar gas and ion source gas or vapor are, for example, supplied through a gas inlet port of the ion source chamber (arc chamber) and thermoelectrons are emitted from the tungsten filament in the chamber. Further, the direction of movement of the emitted thermoelectrons is deflected, and therefore, the probability of collision of the Ar gas and the ion source gas or vapor introduced into the chamber to the thermoelectrons can be increased.
In these conventional ion source chambers, the source of the ions to be irradiated is generally introduced into the arc chamber as gas, or vapor obtained by sublimating the solid as mentioned above. Discharging is made to occur between the filament and an electrode which is opposite thereto, and the thermoelectrons emitted from the filament collide with the gas or vapor to make it ionized, and the ions to be irradiated are obtained and extracted from the chamber.
To achieve the above object, it is necessary to apply the high electric field to the filament and efficiently emit the thermoelectrons. For this reason, tungsten that is a refractory metal is generally used as the material of the filament. In the case of pure tungsten, however, if discharging continues, the temperature of the filament almost rises up to the melting point, and tungsten may be crystallized when the temperature drops after stop of discharging. In a next discharging, the temperature of the crystalline grain boundary rises.up locally and therefore the filament is broken.
For this reason, adding a trace amount of metals such as Al, Si, K and the like to pure tungsten and raising the recrystallization temperature of tungsten to improve its strength at a high temperature has been conventionally executed.
Such a melting point raising technique of adding a trace amount of impurities to the tungsten filament is also used for a filament of a fluorescent lamp or the like. In the case of the filament for the ion generation apparatus chamber, however, the filament is in direct contact with the specific material gas that is introduced into the chamber as the ion source. Many specific material gases generally have corrosiveness and reactivity. Therefore, the environment of use of the filament for the ion generation apparatus chamber is more severe than that of the filament of the fluorescent lamp used generally in an inert gas atmosphere. For this reason, there is a problem that the lifetime of the filament for the ion generation apparatus chamber is short.
Further, when a partial pressure of desired gas is low, it is necessary to increase the filament current to obtain a desired ion current. However, even if a trace amount of metals such as Al, Si, K and the like are added to raise the recrystallization temperature of tungsten, inconvenience such as the breakage of the filament occurs as a consequence of the recrystallization of tungsten.
Even when the filament is not broken, the impurities segregate the recrytalline grain boundary, which prevents the thermoelectrons from being emitted from the filament.
The first object of the present invention is to provide an ion generation method and an ion irradiation method that can execute the ionization of In stably and safely.
The second object of the present invention is to provide a filament which has a long life and allows flow of a high filament current and which can reduce variation in the filament current, and also provide an ion generation apparatus using the filament.
To achieve the above objects, the ion generation method according to a first aspect of the present invention comprises the steps of: heating an ion source material composed of a compound of an element of desired ions to be generated and I, to generate vapor of the compound; and generating the ions by discharging the vapor.
The ion irradiation method according to a second aspect of the present invention comprises the steps of: generating desired ions and I ions in the ion generation method above-described; and selectively irradiating the desired ions onto a substrate to be processed.
The first and second aspects are preferably executed as mentioned below.
The above desired ions are the ions of at least one element selected from the group consisting of B, Al, Ga, In, Ti, N, P, As, Sb and Bi.
The above compound is InI.
The step of heating an ion source material comprises a step of heating the InI at a temperature of not lower than 250xc2x0 C. and not higher than 380xc2x0 C., to generate vapor of the InI.
According to the study of the prevent inventor, it is understood that the iodide containing the element of ions which should be generated has no corrosiveness and are stably ionized. It is further understood that the iodide of this kind hardly reacts with oxygen or water and is therefore safe. For this reason, if the iodide containing the element of the ions to be generated is used as the ion source material, the ion generation method and the ion irradiation method that allow the ionization of this element to be executed stably and safely can be implemented as described in the present invention.
The filament according to the third aspect of the present invention comprises: a refractory metal; and at least one of rare earth elements and rare earth metal oxides contained in the refractory metal.
The ion generation apparatus according to the fourth aspect of the present invention comprises: a chamber formed in a shape of a casing; a gas introduction section for introducing gas to generate plasma into the chamber; the filament above-described, arranged in the chamber; a plasma generation section for generating desired ions by generating the plasma of the gas with thermoelectrons emitted from the filament; and an ion outputting section for outputting the ions generated in the chamber outside the chamber.
The ion irradiation apparatus according to the the fifth aspect of the present invention comprises: an ion generation apparatus above-described; and an irradiation chamber which is provided outside the ion generation apparatus and in which ions discharged through an opening portion formed on the ion generation apparatus are irradiated onto a substrate to be processed.
The third to fifth aspects are preferably executed as described below.
The refractory metal is w and at least one of the rare earth elements and the rare earth metal oxides is selected from the group consisting of Re, La, Ce, Y, Re oxides, La oxides, Ce oxides and Y oxides.
The refractory metal is W, at least one of the rare earth elements and the rare earth metal oxides is Re, and the Re is contained in the w at 1% or more and 26% or less.
The refractory metal is W, at least one of the rare earth elements and the rare earth metal oxides is an oxide selected from the group consisting of La oxides, Ce oxides, Re oxides and Y oxides, and a content of the oxide contained in the filament is 5% or less.
In the present invention, thermoelectrons can be stably emitted for a long time and the stable ionization can be implemented without lowering the ion beam current, by using the tungsten filament containing the rare earth oxide. Therefore, a large advantage can be achieved by applying the present invention to, for example, an ion implantation step in the production of a semiconductor apparatus.
In addition, in the present invention, the recrystallization temperature can be made higher, embrittlement caused together with the recrystallization can be restricted and ductility can be kept by using the tungsten filament containing Re. Therefore, the ionization can be stably executed.
Further, the electric resistivity of the filament can be made larger in the present invention. To obtain a predetermined resistance, the diameter of the filament can be made larger than that of the conventional filament. Thus, the strength of the filament cannot only be increased, but also the amount of the emitted thermoelectrons can be remarkably increased. Therefore, as the present invention can contribute to the increase in the amount of beam current, for example, at the time of the lower energy ion implantation, the implantation time can be shortened.